Saddle-point electrons emitted in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>p</mml:mi></mml:math>-He collisions

The ionization of He by 10--45 keV proton impact was investigated employing recoil-ion momentum spectroscopy. For projectile velocities up to 1 a.u, the electron emission pattern in velocity space shows two intense ``banana''-shaped areas similar to the emission pattern previously observed for transfer ionization in collision of $\mathrm{H}{\mathrm{e}}^{2+}$ with He. These structures extend from the target to the projectile velocities and exhibit a similar focusing towards either of the two nuclei. With increasing impact velocity, an accumulation of the electron velocity distribution at the nuclei becomes more pronounced. At 45 keV proton impact a target-cusp-like peak is observed. In addition to this peak the emission pattern shows signatures of the quasimolecular 2p\ensuremath{\pi} promotion, which is characteristic for the process of saddle-point ionization.

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